It is known to position a nozzle block assembly in a steam turbine just downstream of the steam inlet nozzle chamber and upstream of the first row of rotating blades. The nozzle block is an arcuate shaped device that contains a plurality of openings between flow-directing vanes for directing the steam flow in a desired direction toward the first row of blades. The nozzle block must extend completely around the full 360° arc of the rotating blade path, but it is formed in a plurality of individual arc segments to facilitate its manufacture and installation.
FIG. 1 is an end view of a prior art nozzle block 10 that if formed by four 90° segments 12 joined at horizontal and vertical locations around the arc circumference. Each segment 12 includes a plurality of flow-directing vanes 14 defining openings through a central portion of the nozzle block 10 for passing and directing steam flow from the steam inlet nozzle chamber to the first row of rotating blade of a steam turbine. The direction of steam flow is generally parallel to an axis 8 of rotation of a rotor of the steam turbine (i.e. perpendicular to the plane of the view of FIG. 1). The rotor axis 8 of any such turbine defines radial directions perpendicular to the axis 8 (i.e. in the plane of the view of FIG. 1). The term “radially inwardly” is used herein to refer to a direction toward such an axis 8 and the term “radially outwardly” is used herein to refer to a direction away from such an axis 8. The number of nozzle block segments providing steam to the steam inlet nozzle chamber may vary for different turbine designs. Typically there may be four, six or eight steam inlet segments in steam turbines used in electrical power generating plants.
It is desirable to allow the steam from a given inlet nozzle to flow into only a partial arc of the rotating blades to accommodate low power operation. To accomplish this flow separation, flow blockage devices are positioned at predetermined locations around the arc of the nozzle block 12 to separate the circumferential flow of steam. Such blockage devices are preferably placed at the location of a joint between two segments 12 to simplify the manufacturing process. Conversely, at joint locations where no blockage is desired, the segment joints must accommodate the geometry of the vanes 14 without restriction of the steam flow. FIG. 2 is a partial top view of the joint 16 between segments 12 that is located at the top or bottom vertical position. This position will typically not include a flow blockage device for steam turbines having a total of eight steam inlet segments, such as model BB 44 turbines provided by the assignee of the present invention. To accommodate the angular positioning of the flow-directing vanes 14, the joint 16 is formed as a dogleg joint, i.e. the two segments 12 are joined along two intersecting planes rather than along a single plane. There is an increased cost associated with the manufacturing of such a dogleg joint.
A nozzle block 10 will experience significant flow-induced forces during the operation of the turbine. Some older designs were secured in position by a plurality of bolts. More modern turbines provide for the nozzle block segments 12 to be slid or “rolled” into position within a precisely sized arcuate-shaped opening formed in the turbine nozzle chamber casing. Such nozzle block segments 12 are secured with retaining keys to prevent circumferential movement and with expansion pins to limit axial vibration. FIG. 3 illustrates prior art nozzle block 10 in position within the assembled steam turbine 20. The turbine 20 includes a nozzle chamber casing 22 containing a nozzle chamber passage 24 for receiving steam from steam inlet nozzles (not shown). A rotor shaft 26 is rotatably supported about its axis 8 of rotation within the nozzle chamber casing 22 and supports a first row of blades 28 that are fixed to the shaft 26 and moveably sealed against the nozzle chamber casing 22 by a blade tip seal assembly 30. Nozzle block 10 is positioned within an opening formed in the nozzle chamber casing 22 between the nozzle chamber passage 24 and the first row of blades 28. Nozzle block 10 includes a plurality of vanes 14 for directing the steam flow into the blades 28. Nozzle block 10 is forced against the nozzle chamber casing 22 along an outer flange 32 and an inner flange 34 by the pressure of the steam flow to provide a seal against steam flow bypassing vane 14. The terms outer and inner are used herein to denote relative positions along a radius extending from the axis 8 of rotation of the rotor 26.
The nozzle block 10 of FIGS. 1–3 is installed into turbine 20 by sliding the respective segments 12 into an arcuate-shaped opening formed in the nozzle chamber casing 22. This is a difficult process because the clearances are tight and because the segment 12 must be rotated about the rotor axis as it is slid into position within the arcuate-shaped opening. Upper and lower casing halves of the turbine 20 are separated along a horizontal flanged joint and are typically supported on the turbine deck for service operations with the flanged joint side being upward to expose the interior components of the turbine. Each of four 90° segments 12 is then slide into or out of position. The segment arc-lengths are limited to ninety degrees in order to facilitate the installation process. Furthermore, the dimensions of the nozzle block 10 and those of the receiving opening in the nozzle chamber casing 22 are selected to allow a relatively generous clearance of 0.004–0.008 inches to facilitate the installation/removal processes. Even with this large clearance the installation and removal processes are slow and risky and occasionally result in a segment 12 becoming stuck or damaged. Moreover, to limit the level of flow-induced vibration of the nozzle block 10 during turbine operation, and in particular during partial loading of the turbine 20, a plurality of expansion pins 36 are used to urge the nozzle block 10 against the flanges 32, 34. The expansion pins 36 are selected to have a coefficient of thermal expansion that is greater than surrounding materials so that they expand and provide a stabilizing force against the nozzle block 10 when the turbine is at operating temperature, while at the same time shrinking at room temperature to allow the nozzle block 10 to be installed/removed without making contact with the pins.